Modifying colloidal gold particles with DNA is a new interesting approach in the development of genetic biosensors. Normally the modification is designed to consist of a covalent gold-sulfur bond mediated by a thiol group on one end of a single-stranded oligonucleotide. Here we investigate to what extent the binding actually consists of only the sulfur bridge or if some other nonspecific binding mechanism is present as well. We report on an electrophoresis study showing high amounts of strong, non-thiol-mediated (nonspecific) binding of both single- and double-stranded DNA to gold nanoparticles. Interestingly, even the double strands, lacking interacting groups from the exposed bases of single-stranded DNA, interact nonspecifically with the gold particles. We suggest the mechanism for this to be ion-induced dipole dispersive interactions, where the negatively charged phosphate groups on the DNA induce dipoles in the highly polarizable gold particles. Moreover, we show that particles with nonspecifically adsorbed DNA can be separated from the specifically modified and unmodified ones by gel electrophoresis.
